1. Field of the Invention
The present invention relates to a device for the output regulation of at least one variable displacement pump driven by a motor, delivering into a working line, and to a method of regulating the output of a variable displacement pump, driven by a motor, delivering into a working line.
2. Discussion of the Prior Art
There is shown in FIG. 2 a device for output regulation, such as is known from German patent 44 05 234, upon which the claim preamble is based. The hydrostatic variable displacement pumps 1 and 2 illustrated in FIG. 2, such as for example axial piston pumps of swash plate construction, are connected via respective working lines 3 to respective not-illustrated consuming devices such as for example the travel drive and the excavator drive motor of an excavator, and are connected via respective suction lines 4 to a tank 5. The two variable displacement pumps 1, 2 are driven by means of a common drive motor 6, such as for example an internal combustion engine, and rotate with the same speed of rotation. For varying their displacements there are provided respective setting devices, each comprising two single acting hydraulic setting cylinders 7, 8.
A setting piston 9 is displaceably arranged in each setting cylinder 7, 8 and is coupled via a piston rod 10 with a setting member 11, 12 of the respective variable displacement pump 1 or 2. Each setting piston 9 bounds with its larger piston surface, away from the piston rod 10, a pressure chamber 13 in the respective setting cylinder 7 or 8. The pressure chambers 13 of the setting cylinders 7 are connected via respective setting pressure lines 14 to the working line 3 associated with the respective variable displacement pump 1 or 2. From these working lines 3 there lead respective pressure lines 15 to the pressure chambers 13 of the setting cylinders 8, in which pressure chambers there are arranged respective compression springs 16 which act upon the swash plate of the variable displacement pump 1 or 2 in the direction of maximum displacement, if appropriate aided by the working pressure taken off from the respective working line 3 via the respective pressure line 15, by way of the piston 9, the piston rod 10 and the setting member 12. The piston surfaces of the setting pistons 9 away from the piston rod 10 in the setting cylinders 7 are greater than those of the setting pistons 9 arranged in the setting cylinders 8, so that working pressure which builds up as a setting pressure during the operation of the variable displacement pumps 1, 2 in the pressure chambers 13 of the setting cylinders 7, via the setting pressure lines 14, adjusts the swash plates of the respective variable displacement pumps 1, 2 in the direction of minimum displacement, by way of the piston 9 and the piston rod 10. Respective adjustable stops 17 in the pressure chambers 13 of the setting cylinders 7 serve to limit the maximum displacements of the variable displacement pumps 1, 2 to different values.
Respective regulation valves 18 are arranged in the setting pressure lines 14 and divide these lines into respective setting pressure line sections 19, 20 which lead to the respective working line 3 and to the respective setting cylinder 7. Each regulation valve 18 is constituted as a continuously adjustable 3/2-port directional control valve and has two working connections to the setting pressure line sections 19 and 20 and a working connection to a relief line 21 leading to the tank 5, into which there opens a leakage oil line 22 of the respective variable displacement pump 1 or 2. In the initial disposition shown in FIG. 1, the working connection to the setting pressure line section 19 is blocked, whilst the other working connections are open. From each setting pressure line section 19, a control pressure line 23 leads to a control connection of the associated regulation valve 18 and makes possible its adjustment in the direction of an end position in which the working connections to the setting pressure line section 20 and the relief line 21 are open and the remaining working connection is blocked.
The device for the summation output regulation of the two variable displacement pumps 1, 2 includes two electrical control elements 24, two measurement value indicators 25, an electronic control unit 26 and a desired value setting device 27 associated therewith.
The electrical control elements 24 are constituted as force-regulated proportional magnets and are associated each with one of the control valves 18 for the purpose of generation of a counter-pressure acting against the control pressure. They are connected via respective control signal lines 28 with the electronic control unit 26.
The measurement value indicators 25 are constituted as Hall angle sensors which, for the purpose of detecting the respectively set displacement of the variable displacement pumps 1, 2, are associated with the piston rods 10 of the respective setting cylinders 7 and are connected via respective signal lines 29 to the electronic control unit 26.
The electronic control unit 26 is constituted as a microcomputer, in the memory part of which there is stored the characteristic line KL.sub.g illustrated in FIG. 4. Although this characteristic line represents the working pressure p in dependence upon the displacement V of a variable displacement pump, it is designated in the following as a torque characteristic line because of its development which is typical for torque or output regulation. In the present exemplary embodiment, the two variable displacement pumps 1, 2, and thus their torque characteristic lines, are identical so that only one of these characteristic lines is stored in the memory part of the microcomputer 26.
The area p.times.V below the torque characteristic line KL.sub.g is constant for each displacement V, or for each working pressure p, and in the present exemplary embodiment it is equal to the total drive moment of the drive motor 6. The point P on the torque characteristic line KL.sub.g designates the transition from the so-called starting range (vertical section of KL.sub.g) to the regulation range (hyperbolic section of KL.sub.g), and its ordinate value corresponds to the desired value of the counter-pressure at the control valve 18 which is associated with that variable displacement pump 1 or 2 which transmits or is to transmit the total drive moment. Thus, the ordinate value of the point P corresponds also to the maximum torque which can be transmitted by this variable displacement pump; that is, in the present case the total drive moment of the drive motor 6. The desired value of the counter-pressure at the regulation valve associated in each case with the other variable displacement pump is set to zero in order to prevent the drive motor 6 being overloaded through drive of the consuming unit connected to this other variable displacement pump.
The desired value total of these two counter-pressure desired values and thus the sum of the torques of the two variable displacement pumps 1, 2, which may not be greater than the total drive moment of the drive motor 6, is likewise stored in the memory part of the microcomputer 26 and can be divided into desired values of arbitrary level by means of the desired value setting device.
With this system, and in particular with so-called load sensing systems, with which the pressure difference of a consuming unit is regulated to be constant via a directional control valve, there appears the problem that the regulation system is very susceptible to regulation oscillations. These system-determined oscillations are difficult to damp down with conventional hydraulic regulation systems.